Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers, and the power amplifier may often be the primary consumer of power in a personal audio device, and thus, may have the greatest effect on the battery life of the personal audio device. In devices having a linear power amplifier for the output stage, power is wasted during low signal level outputs, because the voltage drop across the active output transistor plus the output voltage will be equal to the constant power supply rail voltage. Therefore, amplifier topologies such as Class-G and Class-H are desirable for reducing the voltage drop across the output transistor(s) and thereby reducing the power wasted in dissipation by the output transistor(s). In such topologies, power consumption is reduced by employing a power supply, typically a charge pump power supply, which has selectable modes of operation based on an amplitude of an audio output signal of the power amplifier, wherein each of the selectable modes provides a different bi-polar supply voltage across power supply rails of the power amplifier.
Performance of personal audio devices with respect to intelligibility can be improved by providing noise canceling using a microphone to measure ambient acoustic events and then using signal processing to insert an anti-noise signal into the output of the device to cancel the ambient acoustic events. Noise canceling approaches often employ an error microphone for sensing a combined acoustic pressure (e.g., combination of desired sound and undesired ambient noise) near a listener's ear drum in order to remove undesired components (e.g., the undesired ambient noise) of the combined acoustic pressure.
A potential drawback of using adaptive noise cancellation with a selectable-mode power supply is that an injected anti-noise signal may have a magnitude that causes clipping of an audio output signal, and the clipping of the output signal may cause instability of other undesirable operation of the adaptive noise cancellation system due to the fact that the system may adapt based on a measured output signal at the error microphone.